Metal-oxide-semiconductor (MOS) devices typically employ protection circuits at the bonding pads (i.e., input and output terminals to prevent damage to the internal circuitry caused by electrostatic discharge (ESD). An electrostatic voltage is discharged by a very rapid current spike with very high peak current but short duration. Usually the voltage level which these protection circuits will withstand is about 3,000 volts. Voltages of this magnitude are easily generated by friction in routine handling, so special precautions have been needed to keep all of the MOS device terminals grounded or shorted together to avoid damage due to these electrostatic voltage buildups.
Therefore, a need exists for improved ESD protection for MOS integrated circuits that can withstand more than 3,000 volts ESD. In particular, it is desirable to provide ESD protection up to 8,000 to 10,000 volts.
U.S. Pat. No. 4,692,781 issued Sep. 8, 1987, entitled "Semiconductor Device With Electrostatic Discharge Protection" by Rountree et al describes an ESD protection circuit for a MOS device. The described protection circuit uses a thick-oxide transistor connected between a metal bonding pad and ground. The channel width of this transistor is chosen to be sufficient to withstand large, short duration current spikes caused by electrostatic discharge. The spacing between a metal-to-silicon contact to the drain of this transistor (which connects to the internal circuitry of the MOS device) and the channel of the transistor (where most of the heat is generated), is chosen to be much larger than usual (e.g., two to three times) to prevent the metal contact from being melted by heat propagating along the silicon surface during the EDS current spike. A similar spacing feature is also applied to circuits for output pads, or circuits using diode protection devices.
The above described protection device employs two transistors as part of the protection device. As a result, the size of the overall integrated circuit is increased, as well as the complexity and manufacturing costs.